Electronic devices, including mobile computing and/or communication devices, are becoming smaller thereby driving the weight and size of data storage devices down, while requiring large storage capacity in the terabyte range and low power consumption. An increasing storage capacity would require the need for increased precision in tracking the movement of the read/write head.
Data storage devices, such as hard disk drives (HDDs), employ servo systems for tracking and controlling the movement of the read/write head. Conventional servo systems employ embedded servo where the servo information runs radially from the inner diameter (ID) to the outer diameter (OD) of the disc in a series of “servo wedges” interspersed with data. Therefore, the servo information is only detected when the read/write head moves over these servo wedges. In between the servo wedges, no servo information is received by the head. Conventional servo systems typically employ ABCD servo-burst-signal pattern, from which the position error signal (PES) is determined. In systems employing the ABCD servo-burst-signal pattern, position information is derived, for example, from the relative amplitudes of the A burst to the B burst. Furthermore, conventional servo systems also employ frequency servo schemes for PES demodulation, where the position information is derived from the relative amplitudes of one frequency to another frequency of a servo signal.
FIG. 1A shows a general schematic block diagram for a conventional servo control system 100. The servo controller 106 is the heart of the system 100 that provides a control signal to the plant 110. Plant input noise di is injected at summer 108 and could include electronics noise in the circuits of the system 100. The plant 110, in a hard disk drive (HDD), embodies the VCM (voice coil motor) and arm which control the position of the head (e.g. read/write head). Plant output noise do is injected into the system 100 at summer 112 before the signal y is compared to the reference signal, ref., at summer 102, and includes vibrations in the system due to windage, NRRO (non-repeatable run-out) and external shock and vibe. The reference signal compared at summer 102 represents the signal the controller 106 is attempting to follow and could be an offset of the head due to a drift or disturbance. The signal pest is the true position error signal representing the actual difference between the track center, and the head position. The true PES signal, pest, is contaminated or affected with additional noise n at summer 104 before it is fed as input into the controller 106, closing the servo control loop. The source of noise n at summer 104 could be due to all the usual sources of noise in a recording system such as electronics noise, thermal noise, or media noise. On adding this noise to pest, the pes signal that drives the controller 106 is obtained.